Flat panel detector for radiation imaging and pixel for use therein

ABSTRACT

A flat panel detector for radiation imaging includes an array of pixels arranged in rows and columns. Gate lines interconnect the pixels of each row while source lines interconnect the pixels of each column. A radiation transducer is disposed over the pixel array. Each pixel includes a TFT switch having its gate electrode connected to a gate line and its source electrode connected to a terminal that is connectable to ground. The drain electrode of each TFT switch is connected to the pixel electrode of the pixel. The pixel electrode and a bottom electrode connected to a source line constitute a storage capacitor. When the TFT switch is biased and the pixel electrode is positively charged, the pixel electrode discharges through the TFT switch and onto the grounded terminal. This results in the release of negative charge held by the bottom electrode onto the source line, which is sensed by a charge amplifier connected to the source line.

TECHNICAL FIELD

The present invention relates to imaging systems and in particular to aflat panel detector for radiation imaging and to a pixel for use in aflat panel detector.

BACKGROUND ART

Flat panel detectors for use in x-ray radiation imaging systems areknown. Examples of these pixel sensor arrays can be found in U.S. Pat.Nos. 5,184,018 and 5,381,014. One such type of flat panel detectorincludes a thick amorphous selenium film (a-Se) on a two-dimensional TFTswitch array. The TFT switches are arranged in rows and columns to forma two-dimensional imaging system. Gate lines interconnect the TFTswitches in each row while source lines interconnect the TFT switches ineach column. The thick selenium film is deposited directly on top of theTFT switch array and a top electrode is deposited on the selenium film.

When x-rays are incident on the selenium film and the top electrode isbiased with a high voltage, electron-hole pairs are separated by theelectric field across the thickness of the selenium film. The holeswhich are driven by the electric field move toward the pixel electrodes(i.e. the drain electrodes of the TFT switches) and accumulate. Thisresults in a charge being held by the pixel electrodes which can be usedto develop an x-ray image. The charge held by the pixel electrodes canbe read by supplying a pulse to each gate line. When a gate linereceives a pulse, the TFT switches in the row turn on, allowing thesignal charges on the pixel electrodes to discharge through the TFTswitches on to the source lines. Charge amplifiers connected to thesource lines sense the charge and provide output voltage signalsproportional to the charge and hence, proportional to the radiationexposure on the selenium film.

Although this pixel design is satisfactory, alternative designs arecontinually being sought. It is therefore an object of the presentinvention to provide a novel flat panel detector for radiation imagingand a pixel for use therein.

DISCLOSURE OF THE INVENTION

According to one aspect of the present invention there is provided apixel for a radiation imaging flat panel detector comprising:

a radiation transducer to be exposed to incident radiation;

a pixel electrode on one side of said radiation transducer to accumulatea positive charge proportional to the exposure of said radiationtransducer to radiation;

a second electrode separated from said pixel electrode by a dielectricand connected to a source line, said pixel and second electrodesconstituting a storage capacitor, said second electrode developing anegative charge approximately equal to the magnitude of the positivecharge accumulated by said pixel electrode; and

a semiconductor switch coupled between said pixel electrode and aterminal connectable to ground, said semiconductor switch beingresponsive to a gating signal to connect electrically said pixelelectrode to said terminal connectable to ground to discharge said pixelelectrode thereon, said bottom electrode releasing said negative chargeon said source line when said pixel electrode discharges to allow thecharge to be detected.

According to another aspect of the present invention there is provided aflat panel detector for radiation imaging comprising:

a radiation transducer including a radiation conversion layer and anelectrode on one side of said radiation conversion layer;

an array of pixels arranged in rows and columns on the other side ofsaid radiation conversion layer;

a plurality of sources lines upon which charges accumulated by saidpixels can be sensed, each of said source lines connecting the pixels inindividual ones of one of said rows or columns of said array; and

a plurality of gate lines upon which gating signals are supplied toallow accumulated charges to be sensed, each of said gate linesconnecting the pixels in individual ones of the other of said rows orcolumns of said array, each of said pixels including: a pixel electrodeto accumulate positive charge as a result of hole drift in saidradiation conversion layer occurring upon exposure of said radiationtransducer to radiation and when said electrode is biased; a secondelectrode separated from said pixel electrode by a dielectric andconnected to one of said source lines, said pixel and second electrodesconstituting a storage capacitor, said second electrode developing anegative charge approximately equal to the magnitude of the positivecharge accumulated by said pixel electrode; and a semiconductor switchcoupled between said pixel electrode and a terminal connectable toground, said semiconductor switch being responsive to a gating signal toconnect electrically said pixel electrode to said terminal connectableto ground to discharge said pixel electrode thereon, said bottomelectrode releasing said negative charge on said source line when saidpixel electrode discharges to allow the charge to be detected.

According to yet another aspect of the present invention there isprovided a flat panel detector for radiation imaging comprising:

a radiation transducer including a radiation conversion layer and anelectrode on one side of said radiation conversion layer;

an array of pixels arranged in rows and columns on the other side ofsaid radiation conversion layer and formed on a common substrate;

a plurality of sources lines formed on said substrate and upon whichcharges accumulated by said pixels can be sensed, each of said sourcelines connecting the pixels in individual ones of one of said rows orcolumns of said array; and

a plurality of gate lines formed on said substrate and upon which gatingsignals are supplied to allow accumulated charges to be sensed, each ofsaid gate lines connecting the pixels in individual ones of the other ofsaid rows or columns of said array, each of said pixels including: athin film transistor switch having a drain electrode constituting apixel electrode to accumulate positive charge as a result of hole driftin said radiation conversion layer occurring upon exposure of saidradiation transducer to radiation and when said electrode is biased; abottom electrode separated from said pixel electrode by a dielectricgate insulating layer and connected to one of said source lines, saidpixel and bottom electrodes constituting a storage capacitor, saidsecond electrode developing a negative charge approximately equal to themagnitude of the positive charge accumulated by said pixel electrode;and a source electrode coupled to a terminal connectable to ground, saidthin film transistor switch being responsive to a gating signal toconnect electrically said pixel electrode to said low terminalconnectable to ground to discharge said pixel electrode thereon, saidbottom electrode releasing said negative charge on said source line whensaid pixel electrode discharges to allow the charge to be detected.

According to still yet another aspect of the present invention there isprovided a radiation imaging system including a radiation source and aflat panel detector, said flat panel detector comprising:

a radiation transducer including a radiation conversion layer and anelectrode on one side of said radiation conversion layer;

an array of pixels arranged in rows and columns on the other side ofsaid radiation conversion layer;

a plurality of sources lines upon which charges accumulated by saidpixels can be sensed, each of said source lines connecting the pixels inindividual ones of one of said rows or columns of said array;

a plurality of gate lines upon which gating signals are supplied toallow accumulated charges to be sensed, each of said gate linesconnecting the pixels in individual ones of the other of said rows orcolumns of said array;

an array of charge amplifiers, each connected to one of said sourcelines to detect the charge thereon; and

a gate driver to supply gating signal to said gate lines in successionto allow the charges accumulated by said pixels to be detected on arow-by-row basis, each of said pixels including: a pixel electrode toaccumulate positive charge as a result of hole drift in said radiationconversion layer occurring upon exposure of said radiation transducer toradiation and when said electrode is biased; a second electrodeseparated from said pixel electrode by a dielectric and connected to oneof said source lines, said pixel and second electrodes constituting astorage capacitor, said second electrode developing a negative chargeapproximately equal to the magnitude of the positive charge accumulatedby said pixel electrode; and a semiconductor switch coupled between saidpixel electrode and a terminal connectable to ground, said semiconductorswitch being responsive to a gating signal to connect electrically saidpixel electrode to said terminal connectable to ground to discharge saidpixel electrode thereon, said bottom electrode releasing said negativecharge on said source line when said pixel electrode discharges to allowthe charge to be detected by said charge amplifier.

The present invention provides a relatively simple circuit design withminimal hardware components for detecting the exposure of a radiationtransducer to radiation avoiding the need to discharge a capacitivetransducer through a transistor to a charge amplifier. In addition, whentwo source lines are used with each pixel, the effects of noise on thesource lines are significantly reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described more fullywith reference to the accompanying drawings in which:

FIG. 1 is a schematic of a flat panel detector for radiation imaging inaccordance with the present invention;

FIG. 2 is an equivalent circuit of a pixel forming part of the flatpanel detector of FIG. 1;

FIG. 3 is a top plan view of a pixel forming part of the flat paneldetector of FIG. 1;

FIG. 4 is a cross-sectional view of the pixel of FIG. 3 taken along line4—4;

FIG. 5 is an equivalent circuit of an alternative embodiment of a pixelfor a flat panel detector for radiation imaging; and

FIG. 6 is a schematic of a radiation imaging system incorporating a flatpanel detector.

BEST MODES FOR CARRYING OUT THE INVENTION

Referring now to FIG. 1, a flat panel detector for radiation imaging isshown and is generally indicated by reference numeral 20. The flat paneldetector includes a plurality of pixels 22 arranged in rows and columns.Gate lines 24 formed of Chromium interconnect the pixels 22 of each rowwhile source lines 26 interconnect the pixels 22 of each column. Thegate lines 24 lead to a gate driver circuit 28 which provides pulses tothe gate lines in succession in response to input from a control circuit29. The source lines 26 lead to charge amplifiers 30 which in turn areconnected to an analog multiplexer 32. The analog multiplexer providesimage output which can be digitized to create a digitized radiationimage in response to input from the control circuit 29.

FIG. 2 shows an equivalent circuit of one of the pixels 22. As can beseen, the pixel includes a radiation transducer C_(SE) coupled to node Cand to a high potential voltage source 74 in the order of 3 kV. Astorage capacitor C_(ST) is also coupled to node C as well as to sourceline 26. The drain electrode 40 of a thin film transistor (“TFT”) switch38 is also coupled to node C and defines the pixel electrode of pixel22. The source electrode 42 of TFT switch 38 is coupled to one of thegate lines 24 while the gate electrode of the TFT switch 38 is coupledto another of the gate lines 24.

Referring now to FIGS. 3 and 4, one of the pixels 22 formed inaccordance with the present invention is better illustrated. A bottomelectrode 52 is formed on a glass substrate 50 together with the sourceline 26 and is electrically connected to the source line 26. Adielectric layer 53 overlies the source line 26, bottom electrode 52 andsubstrate 50. The gate line 24 is deposited on dielectric layer 53 andis formed of Chromium. A gate insulating layer 54 formed of SiO₂ orSiN_(x) overlies the gate line 24 and dielectric layer 53. Deposited onthe gate insulating layer 54 above the gate line 24 is a semiconductormaterial layer formed of Cadmium Selenide (CdSe) defining the channel 56of the TFT switch 38. Contacting the channel 56 are the drain and sourceelectrodes 40 and 42 respectively of the TFT switch 38. A passivationlayer 58 formed of SiO₂ or Si covers the portion of the channel 56 notcovered by the drain and source electrodes.

Above the TFT switch array is the radiation transducer C_(SE). Theradiation transducer is in the form of a selenium (Se) radiationconversion layer 70 having a thickness of approximately 300 μm to 500μm. Above the radiation conversion layer is a top electrode 72 formed ofIn, Al or Au. The top electrode 72 is coupled to a high potentialvoltage source 74 to provide the necessary bias to the radiationtransducer C_(SE). The top electrode 72 and the drain electrode 40 formthe electrodes of radiation transducer C_(SE) while the drain electrode40 and bottom electrode 52 form the electrodes of storage capacitorC_(ST).

Although only one pixel 22 is shown, it should be realized that each ofthe pixels 22 in the array are identical and that the pixels are formedon the substrate 50 simultaneously by depositing appropriate layers onthe substrate and etching them as required.

In operation, the top electrode 72 is biased to a high potential byvoltage source 74 and the flat panel detector 20 is exposed toradiation, resulting in an electric field being created in the radiationconversion layer 70 which causes electrons to move towards the topelectrode 72 and holes to move towards the drain or pixel electrodes 40.The majority of the holes drift to the drain electrodes where positivecharges are accumulated.

While the flat panel detector 20 is being exposed to radiation, the gatelines 24 are suitably maintained to keep the TFT switches 38 in anoff-state. With the TFT switches off, as the drain electrodes 40accumulate positive charges, negative charges are pulled on to thebottom electrodes 52 from the charge amplifiers 30 via the source lines26 so that equal and opposite charges appear at the electrodes 40 and 52constituting the storage capacitors C_(ST).

After the flat panel detector 20 has been exposed to radiation and it isdesired to create an image, a bias is applied to each gate line 24 insuccession by the gate driver 28 in response to input from the controlcircuit 29. When a bias is applied to a gate line 24, all of the TFTswitches 38 connected to that gate line turn on. This allows the chargesheld by the bottom electrodes 52 of those pixels to be sensed by thecharge amplifiers 30 connected to the source lines 26 extending to thosepixels 22. Thus, by biasing the gate lines 24 in succession, a radiationimage can be created on a row-by-row basis. The manner in which thecharge held by bottom electrodes 52 is sensed will now be described withreference to a single pixel 22.

When the bias is applied to the gate line 24, the TFT switch 38 isconditioned to an on-state. With the TFT switch on, the drain electrode40 is electrically coupled to the source electrode 42 and hence, to thegate line 24 associated with another row of pixels 22 through the TFTswitch 38. At this stage, the other gate line 24 is grounded andtherefore, is at a low potential.

When this occurs, the charge held by the drain electrode 40 isdischarged through the TFT switch 38 on to the gate line 24 at ground.As this occurs, the negative charge on the bottom electrode 52 pulledfrom the charge amplifier 30 via source line 26 is released. This changein charge on the source line 26 is sensed by the charge amplifier 30.Since the charge on the bottom electrode 52 is proportional to theexposure of the radiation transducer C_(SE) to radiation, the change incharge sensed by the charge amplifier 30 is also proportional to theexposure of the radiation transducer to radiation. The charge amplifier30 in turn generates output representative of the exposure of theradiation transducer C_(SE) to radiation and conveys the output to theanalog multiplexer 32.

Referring now to FIG. 5, another embodiment of a pixel for use in a flatpanel detector for radiation imaging is shown and is generally indicatedby reference numeral 122. The pixel 122 is very similar to that shown inthe previous embodiment with the exception that the source electrode 142of TFT switch 138 is connected to a second source line 126′, rather thanto the gate line 124. In this embodiment, the source lines 126 and 126′associated with the pixel 122 lead to a balanced charge amplifier 130.Because each pixel has two source lines, noise contributed by one sourceline 126 cancels the noise contributed by the other source line 126′reducing the overall effect of noise. Also, the use of two source lineshelps to reduce the contribution of even harmonics generated by the TFTswitch 138 as well as to reduce the effects of odd harmonics.

During operation of the pixel 122, after the flat panel detector hasbeen exposed to radiation and the bottom electrode is charged, when thebias is applied to the gate line 124, the charge held by the drainelectrode 140 is discharged through TFT switch 138 on to source line126′ where it is sensed by charge amplifier 130. At the same time, thecharge on the bottom electrode 52 is released. The resulting change incharge on the source line 126 is also sensed by the charge amplifier130. The balanced charge amplifier 130 detects the input from the sourcelines 126 and 126′ and generates an output voltage proportional to theexposure of the pixel 122 to radiation.

FIG. 6 shows an x-ray imaging system 200 for taking a radiation image ofan object 202. The x-ray imaging system 200 includes a radiation source204 and a flat panel detector 206. The flat panel detector can includepixels of the type shown in FIG. 2 or 5. As can be seen, when an x-rayimage of an object is to be taken, the object 202 is placed between theradiation source 204 and the flat panel detector 206 and the object isexposed to x-ray radiation. X-ray radiation passing through the object204 contacts the flat panel detector 206 allowing for an image of theobject to be taken in the manner described previously.

As should be appreciated by those of skill in the art, the presentinvention allows charges accumulated by the pixel electrodes to besensed without discharging the charge through TFT switches to the chargeamplifiers. Also, when two source lines are used in each pixel,electronic noise is reduced. Although reference has been made tospecific materials forming the various components of the pixels, thoseof skill in the art will appreciate that other suitable materials can beused. Those of skill in the art will also appreciate that variations andmodifications may be made to the present invention without departingfrom the scope thereof as defined by the appended claims.

What is claimed is:
 1. A pixel for a radiation imaging flat paneldetector comprising: a radiation transducer to be exposed to incidentradiation; a pixel electrode on one side of said radiation transducer toaccumulate a positive charge generally proportional to the exposure ofsaid radiation transducer to radiation; a second electrode separatedfrom said pixel electrode by a dielectric and connected to a sourceline, said pixel and second electrodes constituting a storage capacitor,said second electrode developing a negative charge approximately equalto the magnitude of the positive charge accumulated by said pixelelectrode; and a semiconductor switch coupled between said pixelelectrode and a terminal connectable to ground, said semiconductorswitch being responsive to a gating signal to connect electrically saidpixel electrode to said terminal when said terminal is connected toground to discharge said pixel electrode thereon, said second electrodereleasing said negative charge on said source line when said pixelelectrode discharges to allow the charge to be detected.
 2. A pixel asdefined in claim 1 wherein said semiconductor switch is in the form of athin film transistor having a drain terminal connected to said pixelelectrode, a gate terminal connected to a gate line and being responsiveto said gating signal, and a source terminal connected to said terminalconnectable to ground.
 3. A pixel as defined in claim 2 furthercomprising a charge amplifier connected to said source line fordetecting the release of said negative charge.
 4. A pixel as defined inclaim 3 wherein said source terminal is connected to a second sourceline, said second source line also being connected to said chargeamplifier.
 5. A pixel as defined in claim 2 wherein said source terminalis connected to a second gate line.
 6. A flat panel detector forradiation imaging comprising: a radiation transducer including aradiation conversion layer and an electrode on one side of saidradiation conversion layer; an array of pixels arranged in rows andcolumns on the other side of said radiation conversion layer; aplurality of source lines upon which charges accumulated by said pixelscan be sensed, each of said source lines connecting the pixels in anindividual column of said array; and a plurality of gate lines uponwhich gating signals are supplied to allow accumulated charges to besensed, each of said gate lines connecting the pixels in an individualrow of said array, each of said pixels including: a pixel electrode toaccumulate positive charge as a result of hole drift in said radiationconversion layer occurring upon exposure of said radiation transducer toradiation and when said electrode is biased; a second electrodeseparated from said pixel electrode by a dielectric and connected to oneof said source lines, said pixel and second electrodes constituting astorage capacitor, said second electrode developing a negative chargeapproximately equal to the magnitude of the positive charge accumulatedby said pixel electrode; and a semiconductor switch coupled between saidpixel electrode and a terminal connectable to ground, said semiconductorswitch being responsive to a gating signal to connect electrically saidpixel electrode to said terminal when said terminal is connected toground to discharge said pixel electrode thereon, said second electrodereleasing said negative charge on said source line when said pixelelectrode discharges to allow the charge to be detected.
 7. A flat paneldetector as defined in claim 6 wherein said semiconductor switches arein the form of thin film transistors and wherein said pixels and gateand source lines are formed on a common substrate.
 8. A flat paneldetector as defined in claim 7 wherein said terminal connectable toground is a gate line interconnecting the pixels of a different row. 9.A flat panel detector as defined in claim 7 further including an arrayof charge amplifiers, each connected to one of said source lines todetect the charge thereon.
 10. A flat panel detector as defined in claim9 further including a gate driver to supply gating signal to said gatelines to allow the charges accumulated by said pixels to be detected ona row-by-row basis.
 11. A flat panel detector as defined in claim 7wherein said terminal connectable to ground is a second source lineassociated with said pixel, said charge being detected by sensing bothsource lines.
 12. A flat panel detector as defined in claim 11 furtherincluding an array of charge amplifiers, each connected to both of thesource lines interconnecting an individual column of said array todetect the charges thereon.
 13. A radiation imaging system including aradiation source and a flat panel detector, said flat panel detectorcomprising: a radiation transducer including a radiation conversionlayer and an electrode on one side of said radiation conversion layer;an array of pixels arranged in rows and columns on the other side ofsaid radiation conversion layer; a plurality of sources lines upon whichcharges accumulated by said pixels can be sensed, each of said sourcelines connecting-the pixels in an individual column of said array; aplurality of gate lines upon which gating signals are supplied to allowaccumulated charges to be sensed, each of said gate lines connecting thepixels in an individual row of said array; an array of chargeamplifiers, each connected to one of said source lines to detect thecharge thereon; and a gate driver to supply gating signal to said gatelines in succession to allow the charges accumulated by said pixels tobe detected on a row-by-row basis, each of said pixels including: apixel electrode to accumulate positive charge as a result of hole driftin said radiation conversion layer occurring upon exposure of saidradiation transducer to radiation and when said electrode is biased; asecond electrode separated from said pixel electrode by a dielectric andconnected to one of said source lines, said pixel and second electrodesconstituting a storage capacitor, said second electrode developing anegative charge approximately equal to the magnitude of the positivecharge accumulated by said pixel electrode; and a semiconductor switchcoupled between said pixel electrode and a terminal connectable toground, said semiconductor switch being responsive to a gating signal toconnect electrically said pixel electrode to said terminal when saidterminal is connected to ground to discharge said pixel electrodethereon, said second electrode releasing said negative charge on saidsource line when said pixel electrode discharges to allow the charge tobe detected by said charge amplifier.
 14. A radiation imaging system asdefined in claim 13 wherein said terminal connectable to ground is agate line interconnecting the pixels of a different row.
 15. A radiationimaging system as defined in claim 14 wherein said semiconductorswitches are thin film transistors.